A gas turbine engine, such as a turbo fan engine for an aircraft, includes a fan section, a compression section, a combustion section and a turbine section. An axis of the engine is centrally disposed within the engine and extends longitudinally through these sections. The primary flow path extends axially through the sections of the engine. A secondary flow path for extends parallel to and radially outward of the primary flow path.
The fan section includes a rotor connected to a plurality of radially extending fan blades. The fan blades extend through the flow path and interact with the air and transfer energy between the fan blades and the air. A fan case acts as a stator and circumscribes the rotor and fan blades in close proximity to the tips of the fan blades.
During operation, the fan draws the air into the engine. The fan also raises the pressure of the air drawn along the secondary flow path, thus producing useful thrust. The air drawn along the primary flow path into the compressor section is compressed. The compressed air is channeled to the combustion section where fuel is added to the compressed air and the air/fuel mixture is burned. The products of combustion are discharged to the turbine section. The turbine section extracts work from these products to power the fan and compressor section. Energy from the products of combustion not needed to drive the fan and compressor contributes useful thrust.
In order to reduce weight, the fan blades in some gas turbine engines are hollow. According to US 2007/0128042, each fan blade may be made by combining two separate halves. Each half may include a plurality of cavities and ribs machined out to reduce the weight while forming a structurally sound internal configuration. One half forms the pressure side wall and the other half forms the suction side wall. When the halves are joined, the pressure side wall and the suction side wall are separated and supported by the ribs to form a hollow fan blade. The hollow fan blade is then subjected to forming operations at extremely high temperatures at which time it is given an airfoil shape and geometry. The side walls may be contoured and curved to form an airfoil.
Hollow and therefore lighter fan blades improve thrust-specific fuel consumption (TSFC). Another way to lighten a fan blade is to hollow out cavities in a solid structure and adhere on a cover. For a titanium fan blade, one option is a titanium cover. However, titanium covers are hard to form due to their stiffness. Even with hot forming, the thickness of the cover that can be formed is limited. Since the thickness of the cover sets the wall thickness for much of the side the cover is on, that compromises the stiffness of the blade and a thin cover is more likely to buckle under bird impact. Another problem with a thicker cover is that it is also hard to make it conform or be flush to the blade to eliminate or reduce low dams and waterfalls, that result from variations in the surfaces of blade body and cover. Dams and waterfalls can be eliminated by sanding epoxy around the outer perimeter of the cover, but a cover that generates dams and waterfalls will increase the time needed to sand the epoxy and may result in a shape different from the design intent. Thus, a more conformable cover that generates lower dams and waterfalls is needed.
Another potential problem with a cover is that the glue area may be insufficient to hold on to the cover under all conditions. With variation between suppliers and operators in bond preparation during manufacture, increasing the glue area for the cover will help mitigate the risk that a cover that did not undergo an ideal bond preparation procedure will still stay on under all conditions.